1. Field of the Invention
The present invention relates to a brake pressure control apparatus for an anti-lock braking system in automobiles, and more particularly to a brake pressure control apparatus for an anti-lock braking system in automobiles, which has a simple construction and easily operates.
2. Prior Arts
Generally, an anti-lock braking system (ABS) is used for preventing the wheels of the automobile from "locking" in the event of a sudden stop of the automobile. The term "locking of the wheels" means that the rolling wheels of the automobile are stopped by the brake pressure applied to the wheels when a driver suddenly puts on the brake while driving the automobile. When the wheels are subjected to this type of locking condition, the wheels slip toward the running direction due to the inertia force of the vehicle so that the frictional force between the wheels and the road surface may be reduced. For this reason, the braking distance may become longer and steering the vehicle may be impossible, thereby causing fatal accidents.
In order to prevent these types of accidents, an anti-lock braking system is provided in the vehicle. The anti-lock braking system increases, maintains, and reduces the brake pressure applied to the wheels rapidly and repeatedly so as to avoid wheel locking, thereby preventing fatal accidents.
Generally, the ABS comprises a valve system operated by a hydraulic pump or an electric signal in order to increase, maintain and reduce the brake pressure applied to the wheels, a sensor for sensing the RPM of the wheels, and a control unit which operates the ABS according to a predetermined algorithm.
In the ABS as mentioned above, the first step wherein the brake pressure applied to the wheels is increased is called a pressure increasing mode, the second step wherein the brake pressure applied to the wheels is constantly maintained is called a pressure maintaining mode and the third step wherein the brake pressure applied to the wheels is reduced is called a pressure reducing mode.
In the conventional ABS, the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode are performed by operating a solenoid valve.
FIGS. 9A-9C show a solenoid valve 500 under the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode of the conventional ABS, respectively.
As shown in FIGS. 9A-9C, solenoid valve 500 includes a cylindrical valve body 510 and a cover 520 attached to the lower end of cylindrical valve body 510 for preventing the pressure fluid from being leaked. Cylindrical valve body 510 has a first port 512 connected to a wheel cylinder (not shown), a second port 560 through which the pressure fluid circulates to a pump (not shown), a first chamber 516 formed above second port 560, and a second chamber 518 formed below second port 560.
An upper valve seat 550 having a pressure fluid inlet 552 is provided at the upper end of first chamber 516 and a lower valve seat 560 having a pressure fluid outlet 564 is provided at the lower end of first chamber 516. A cylinder 558 is disposed between upper valve seat 550 and lower valve seat 560. At the upper and lower ends of cylinder 558, there are provided first and second balls 554 and 562, respectively. First ball 554 is in contact with upper valve seat 550, and second ball 562 is in contact with lower valve seat 560.
In addition, first and second springs 556 and 566 are accommodated in cylinder 558 in such a manner that first and second springs 556 and 566 can elastically support first and second balls 554 and 562, respectively. Between first and second springs 556 and 566, there is disposed a head portion 534 of a movable plunger 532. Head portion 534 pushes first spring 556 when plunger 532 moves upwards, thereby pressure fluid inlet 552 of upper valve seat 550 is closed by first ball 554.
Second chamber 518 of valve body 510 includes an armature 530 securely coupled to one end of plunger 532, a bush 542 inserted in the upper end portion of second chamber 518 in order to guide the movement of plunger 532, and a solenoid 540 which applies the magnetic force to armature 530 thereby moving armature 530 upwards. Between armature 530 and the underside of lower valve seat 560, a third spring 576 is disposed for elastically supporting armature 530.
The conventional solenoid valve having the above structure operates as follows.
When a driver suddenly puts on the brake, the speed of the vehicle suddenly decreases. At this time, a speed sensor (not shown) attached to the wheel of the vehicle senses the decreased speed and then sends the data to an electrical control unit (ECU, not shown). Upon receiving the data from the speed sensor, the ECU compares value of the data with a predetermined value which is preset in the ECU. If the value of the data exceeds the predetermined value, the ECU operates the ABS according to a predetermined algorithm. That is, the brake pressure applied to the wheels of the vehicle is continuously increased (the pressure increasing mode), constantly maintained (the pressure maintaining mode) or reduced (the pressure reducing mode) in accordance with the predetermined algorithm of the ECU.
In the pressure increasing mode, as shown in FIG. 9A, the ECU operates a hydraulic pump (not shown) so that the pressure fluid flows into pressure fluid inlet 552 from the hydraulic pump while pushing first ball 554 downwards. Then, the pressure fluid that has flowed into pressure fluid inlet 552 is applied to the wheel cylinder connected to the wheels through first port 512, so that the brake pressure to the wheels increases.
Next, electric power is applied to solenoid 540 so that magnetic power is generated by solenoid 540. As the magnetic power is generated, armature 530 moves upwards while overcoming the bias force of third spring 576.
When armature 530 moves upwards, plunger 532 securely inserted in armature 530 also moves upwards while pushing first spring 556 upwards. Therefore, as shown in FIG. 9B, pressure fluid inlet 552 is closed by first ball 554.
From this state, the pressure fluid does not flow into solenoid valve 500 so that solenoid valve 500 maintains the pressure maintaining mode, wherein the constant brake pressure is applied to the wheels.
While, as the electric power is continuously applied to solenoid 540, armature 530 moves upwards to the underside of bush 542 inserted in the upper end portion of second chamber 518 of valve body 510 as shown in FIG. 9C. Accordingly, second ball 562 rested in lower valve seat 560 is simultaneously pushed by a neck portion 536 of plunger 532 so that second ball 563 moves upwards. At this time, the pressure fluid leaks through a fine gap formed between pressure fluid outlet 564 of lower valve seat 560 and plunger 532, and then, the leaked pressure fluid is exhausted to a fluid tank (not shown) through second port 514. From this state, solenoid valve 500 maintains the pressure reducing mode wherein the brake pressure applied to the wheels of the vehicle reduces.
The conventional solenoid valve rapidly repeats the above pressure increasing mode, the pressure maintaining mode and the pressure reducing mode in a short time period, thereby preventing the wheels from locking even when the driver suddenly puts on the brake.
However, since the conventional solenoid valve requires various elements, the construction of the conventional solenoid valve may be complicated. Therefore, not only controlling the conventional solenoid valve is difficult, but also the cost for manufacturing the conventional solenoid valve is increased.
Further, the first, second and third springs must be manufactured accurately for performing the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode of the solenoid valve precisely. Thus, the manufacturing of the solenoid valve is difficult.
On the other hand, a U.S. patent application entitled "a brake pressure control apparatus for an anti-lock braking system in automobiles" which is manufactured at a low cost with a simple construction and is easily operated, has been filed by the applicant of the present invention and now is pending.
In the above brake pressure control apparatus, the pressure increasing mode, the pressure maintaining mode and the pressure reducing mode are performed by operating a rotary valve without using the solenoid valve.
However, in the above brake pressure control apparatus, the rotary valve does not exactly return to its initial position in which the rotary valve communicates with a wheel cylinder so as to perform the pressure increasing mode, after the first ABS operation has finished.
That is, after the first ABS operation has finished, it is possible for the rotary valve to be stopped at the second position in which the communication between the rotary valve and the wheel cylinder is closed such that the pressure maintaining mode is performed, or the third position in which the rotary valve communicates with a fluid tank so as to perform the pressure reducing mode. In this case, the wheels of vehicle slip toward the running direction momentarily as the second ABS operation starts.
In other words, if the rotary valve is stopped at the first position after the first ABS operation has finished, the braking pressure flows instantly from a hydraulic pump to the wheel cylinder through the rotary valve when the second ABS operation staffs, thereby the normal second ABS operation is performed.
On the contrary, if the rotary valve is stopped at the second position or third position after the first ABS operation has finished, the braking pressure does not instantly flow from the hydraulic pump to the wheel cylinder when the driver suddenly puts on the brake since the communication between the rotary valve and the wheel cylinder is closed. Accordingly, the second ABS operation is momentarily delayed, and thereby the wheels of the vehicle slip in the running direction.
This type of slip confuses the driver and causes accidents.